The invention described herein relates to nuclear reactors and more particularly to improved control rod guide thimbles designed to minimize wear of control rods during reactor operation.
Commercial nuclear reactors of conventional design includes a core composed of multiple fuel assemblies. Each fuel assembly includes an array of fuel rods held in spaced relationship with each other by grids of egg-crate configuration spaced along the fuel rod length. Control rod guide tubes or thimbles interspersed among the fuel rods provide a pathway for control rods which reciprocate therein to control the fission process which takes place in the reactor. These guide thimbles are immovably connected to the longitudinally spaced grids and these components thus form the basic structural framework for each fuel assembly. To extract heat from the core, coolant is circulated upwardly in contact with the heat generating fuel rods from which it absorbs heat before being discharged from the reactor. Since the control rod guide thimbles are open at both lower and upper ends, a portion of the coolant also is circulated therethrough during reactor operation.
As the coolant flows at high velocity and pressure through the guide thimbles, it causes the control rods therein to vibrate and thus move laterally and cyclically into contact with the inner walls of the guide thimbles. It has been found that this action induces wear which occurs where the tip of the control rod interfaces with the guide thimble inner surfaces.
Close examination and analysis of the evidence of wear shows that a scar of triangular geometry occurs in the wear area. This geometry suggests that the control rod experienced lateral vibratory motion which caused wear at the point where point contact is made between the control rod spherical tip and inner walls of the guide thimble. More specifically, as the control rod vibrates, its tip penetrates progressively deeper into the guide thimble wall, and as the control rod slowly advances or moves upwardly during reactor operation, the point of control rod-guide thimble contact also moves upwardly. This effects a change in the wear scar in that it decreases in size and severity because the control rod lateral motion is less as it is withdrawn with changes in reactor reactivity. Nevertheless, inspection of guide thimbles exposed to wear as described above, showed that the guide thimble walls were worn through in some fuel assemblies while others had only partial circumferential wear.
Different designs of guide thimbles have been made to alleviate wear caused by the hydraulically induced vibration of the control rods in the guide thimbles. For example, application Ser. No. 102,046 filed Dec. 10, 1979 by S. Kmonk et al. and assigned to the same assignee as the present invention discloses the use of chromeplated stainless steel wear sleeves within the upper end of the guide thimble at the wear location to help minimize wear caused by the vibrating control rods.